Rechargeable or secondary electrochemical storage devices or batteries have wide-ranging applications and improvement of battery performance is a long-standing goal.
Commercial Li-ion batteries contain a graphitic carbon, or alloy, anode capable of “insertion” (or intercalation) of Lithium ions. Therefore in Li-ion batteries the host material affords a protective barrier against reactions with a liquid electrolyte. The electrolyte of Li-ion batteries generally contains a non-aqueous electrolyte comprising lithium hexafluorophosphate (LiPF6) dissolved in a mixture of carbonate solvents, examples of which include ethylene carbonate (EC), dimethyl carbonate (DMC), diethyl carbonate (DEC), and propylene carbonate (PC). In Li-ion batteries both cathode and anode operate by intercalation of Li ions. This creates an opportunity to improve the kinetics of intercalation by use of an electrolyte additive to modify the interface between the electrolyte and the intercalating compound. The additive acts to modify the chemistry and conductivity of a stable Solid Electrolyte Interphase (SEI) that forms between the active electrode and the electrolyte. Thus in Li-ion batteries the use of additives is well-known as a technique to improve the kinetics and stability of the operation of the cell.
Secondary batteries containing Li metal anodes have been a long-standing goal of battery research for decades, but have failed to gain commercial traction. In secondary Li metal cells the anode operation comprises at least in part plating deposition of metallic lithium or lithium alloy at the anode. Most batteries of this type involve a conversion cathode such as sulfur (S). Additives are known in the Li—S cell, which act to prevent parasitic reactions (primarily the poly-sulphide shuttle reactions).
Yet another class of batteries involves a conventional (“intercalation” or “insertion”) material as a cathode and plating lithium metal at the anode. We will refer to this as an intercalation/plating cell. In this case it is generally acknowledged that it is not possible to form a stable SEI between Li metal and a liquid electrolyte. Consequently the vast majority of this literature is focused on identifying a coating that separates the Li from the electrolyte. Coatings may be polymeric, solid-electrolyte, graphene, core-shell, “yolk-shell” or “pomegranate”, depending on the choice of materials. Such barrier layers to prevent reaction of Li with the electrolyte are also being explored in Li—S batteries. In general for intercalation/plating cells the search for additives to improve cell efficiency has been abandoned. CsPF6 has been proposed as a means to modify the kinetics of the reaction.
LiPF6 has been the primary salt of Li-ion batteries for decades; however its propensity to form HF, PF5, and POF3 in the presence of trace water impurities is expected to be a major problem for Li-metal batteries (Xu, Kang Nonaqueous Liquid Electrolytes for Lithium-Based Rechargeable Batteries Chem. Rev. 2004, 104, 4303-4417). Fortunately, a variety of other salts are known to reversibly plate and strip lithium metal with less reactivity towards water. Some non-limiting examples include lithium bis(oxalato)borate (LiBOB), lithium tetrafluoroborate (LiBF4), and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI). When dissolved in carbonate solvents these salts yield coulombic efficiencies as high as 90+% against Li metal, which is far below the 99+% of LiPF6 in traditional Li-ion (Winter, M. Blends of lithium bis(oxalate)borate and lithium tetrafluoroborate: useful substitutes for lithium difluoro(oxalato)borate in electrolytes for lithium metal based secondary batteries? Electrochimica Acta, 2013, 107, 26-32).
One way to change cell Coulombic efficiency is via electrolyte modification through the use of chemical additives. It is reportedly best to choose additives that are either more readily oxidized at the cathode or reduced at the anode than electrolyte salts and solvents. To this end, additive mixtures are typically selected so that at least one component is reduced at, and therefore passivates, the anode and another is oxidized at the cathode. This is common practice in the Li-ion industry with commercial cells containing up to 3 or 4 individual low percentage (<2% by mass) additives. Non-limiting examples of additives known to improve stability at either the cathode or the anode include carbonate esters such as vinylene carbonate (VC), vinyl ethylene carbonate (VEC), or fluoroethylene carbonate (FEC), sultones (e.g., 1, 3-propene sultone—PS), sulfonates (methylene methane disulfonate—MMDS), or phosphates such as (tris(-trimethyl-silyl)-phosphate (TTSP) (Dahn, J. R. et. al. Ternary and Quaternary Electrolyte Additive Mixtures for Li-ion Cells that Promote Long Lifetime, High Discharge Rate, and Better Safety, JES, 2014, 161, A1261-A1265). There have also been several reports utilizing lithium salts as electrolyte additives. Non-limiting examples include lithium bis(oxalato)borate or lithium difluoro(oxalato)borate (LiDFOB) (EP 2,660,906 A1, U.S. Pat. No. 7,524,579 B1), and lithium methoxide (LiOMe) (US 2011/0006738 A1).
Li-ion (carbonaceous-anode) cells generally utilize additives in fashions that avoid Li metal deposition. For example, one can generate an SEI that alters the rate of electrochemical intercalation and deintercalation of lithium ions to the solid phase of the electrodes (US20110104574 A1, US20150030937 A1), so that the cells can operate at higher C-rates without plating Li metal on the carbon, which is a well-known pitfall of Li-ion batteries. Lithium-Sulfur batteries employing Li metal anodes mainly utilize additives as a means of mitigating the redox-shuttle related to the cathode dissolution and subsequent reduction at the surface of the anode (U.S. Pat. No. 9,160,036 B2) as opposed to improving the reversibility of Li metal in the cell. Alternatively rechargeable Li metal batteries generally coat the anode with a thin layer of polymer, ceramic, or both (e.g., lithium phosphorus oxynitride (LiPON)), which serves as a protective barrier for the Li metal surface. In fact, those practiced in the art have said “because of the enormous challenge involved in stabilizing the Li surface chemically and mechanically through the use of electrolyte additives, the preferred treatment for rechargeable Li-based cells is the use of a solid-electrolyte membrane (US20150050543 A1).”
There is a need for improved rechargeable Li-metal batteries.